High cooling power must be produced in motor vehicles with combustion engines even at low speeds. To do this, fans are used as a rule, which induce heat dissipation when the air stream at low vehicle speeds is no longer adequate to dissipate heat from the radiator. One-piece plastic cooling fans are normally used in passenger vehicles and these are also increasingly being used for heat dissipation in commercial vehicles.
Nowadays, direct-current motors (DC motors) are being used on cooling fans in combustion engines, and these motors drive the fan wheel of the fan, if need be with the interconnection of a coupling. The electric drives used are triggered via power controls, for which a timing of the supply voltage takes place at a frequency above 15 kHz. The timing of the supply voltage occurs via pulse width modulation, whereby the pulse width ratio, i.e., the pulse interval length between the triggering pulses, can be lengthened or shortened, thereby permitting the terminal voltage on the terminals of the electric drives that are used to be varied in a wide range. By varying the terminal voltage at the terminals of the electric drives of the cooling fan, the current consumption of the electric drive or the torque of the electric drive can be regulated or preset. In addition, the speed of the electric drives can be adjusted in wide ranges with the aid of the pulse width modulation of the supply voltage. This is of particular interest in cases where the vehicle is driving at a low speed or idling. Then an increase in the speed of the electric drive of the fan wheel can produce adequate heat dissipation at the radiator of the combustion engine if the cooling via the air stream flowing through the radiator is no longer adequate.
The timing of the supply voltage, which is applied to the terminals of the electrical drives, however, makes the use of free-wheeling diodes as well as capacitor elements necessary. Electrolytic capacitors are used as a rule. The free-wheeling diode makes the free-wheel of the electric drive or the electric drives possible, while the electrolytic capacitors make the free-wheel of the supply line possible. So that the electrolytic capacitors operate without any difficulty also at high temperatures and achieve the required service lives, they are normally large dimensioned in terms of capacitance. In addition, it can be necessary to connect two electrolytic capacitors in parallel in order to achieve the desired smoothing of the residual ripple.
DE 197 32 094 A1 relates to a control circuit for a direct-current motor. An electrolytic capacitor is connected in parallel with the direct-current motor. The control circuit features a free-wheeling diode and a reverse battery protective device, which contains a transistor switch with a diode connected in parallel with it. The reverse battery protective device is switched in the electric circuit of the electrolytic capacitor and the free-wheeling diode. The transistor switch is designed as an n-channel power MOSFET and its drain connection lies at the negative connection of the electrolytic capacitor and at the anode of the free-wheeling diode. The free-wheeling diode's cathode lies at the positive side of the direct-current motor. The source connection is attached at the negative side of the direct-current motor and the gate connection is applied via a resistor at a positive voltage.
DE 197 32 098 A1 also relates to a control circuit for a direct-current motor. The direct-current motor is triggered in a timed fashion and includes a parallel-connected electrolytic capacitor as well as free-wheeling diode. In accordance with this attainment, the control emission is reduced by a choke being attached between the positive motor supply voltage and the positive connection of the electrolytic capacitor, and the free-wheeling diode lying with its cathode between the choke and the electrolytic capacitor and with its anode at the negative side of the direct-current motor.
In current designs of fan triggering for motor cooling fans, excess current detection takes place via the detection of a current in the free-wheeling circuit of the electric drive. The measurement of the current in the free-wheeling circuit of the electric drive occurs indirectly via measurement of the induced voltage at the line inductance of the free-wheeling circuit. Moreover, the detection of current in the free-wheeling circuit can be realized via a shunt, which has the disadvantage that dissipation power is generated and therefore the generation of heat occurs. Furthermore, the measurement of current with the use of a shunt is relatively costly. The methods outlined to measure the induced voltage at the line inductance of the free-wheeling circuit are, along with other parameters, a function of the electric wiring and of the switching behavior of the power transistors. Measuring the induced voltage at the line inductance of the free-wheeling circuit is sensitive to interference irradiation, i.e., from electric fields, because of the low voltages and the high-impedance.